1. Field of the Invention
The invention relates to safety devices and, in particular, to safety devices for use with power driven equipment to prevent opening of an access door on the equipment until rotation of the power driving means has stopped. More particularly, the invention relates to a safety device mounted on meat processing equipment using a solenoid operated locking latch which is controlled by a sensor that is responsive to the back EMF produced by a power drive motor upon the motor rotor approaching a stationary condition.
2. Description of the Prior Art
There are numerous type and constructions of meat processing equipment which use rotating or reciprocating knives, blades, paddles, augers or other components for cutting, slicing, chipping or conveying meat in large quantities. Examples of such equipment are shown in U.S. Pat. Nos. 3,330,318, 3,330,319, 3,599,687 and 4,127,236. Such equipment can be extremely dangerous to its operator due to the high-speed movement of the cutting members and power driven components unless sufficient safety precautions are taken. Accordingly, such equipment usually is enclosed in protective housings or cabinets which completely enclose the moving components, with access being gained thereto only by a cover, lid or door.
However, injuries still occur to the operators of such equipment since an operator will open the access door to perform maintenance on the equipment or to add or subtract the meat product from the equipment to speed production while the equipment is operating. To prevent such improper use of the equipment and the premature opening of the access door, various safety devices have been incorporated into the equipment. Magnetic switches are incorporated into the power supply circuit for the drive motor and are actuated by a permanent magnet mounted on the access door to automatically cut off the power to the drive motor upon opening of the door. Occasionally, equipment operators will bypass such magnetic switches by the placement of a permanent magnet on the switch to maintain the switch actuated even when the door is open. Also, other types of safety devices which have been incorporated into the motor control circuit of the equipment in an attempt to prevent operators from bypassing the safety device are subject to tampering by operators and do not provide a failsafe deterrent.
One problem that still persists with such equipment even if the door safety latch deenergizes the drive motor upon opening of the door, is the continued rotation of the cutting blades or other power driven elements due to the inertia or flywheel effect which remains in the rotating or reciprocating member. Thus, the cutting blade or blade carrying member could continue to move for a considerable period of time, for example, 40 to 50 seconds after the motor has been deenergized upon opening of the door. During this time period in which the rotor of the motor is coasting, an operator still can be injured if he attempts to perform work on the equipment or its contents without waiting until the motor comes to a complete stop.
U.S. Pat. Nos. 2,072,681 and 2,245,012 show a safety device for use with food processing equipment which prevents cutting knives from being driven by the motor except when the access door is closed, and in addition, prevents the equipment cover from being raised to expose the knife except when the motor is stopped. This particular safety device appears to be effective and tamperproof but involves a relatively expensive construction and requires numerous components to achieve the desired results.
U.S. Pat. No. 2,910,317 shows a safety device for a laundry machine which prevents access to the internal laundry tub during rotation of the tub by a solenoid actuated latch. The solenoid remains energized for a predetermined preset time period after the spin drive is cut off to compensate for the coast period of the motor. This type of safety device can cause problems since the time delay mechanism can be tampered with by an operator, and an exact time delay period setting may be difficult to obtain resulting in a too long or too short time delay.
U.S. Pat. No. 2,936,892 discloses another safety door latch for a washing machine using solenoid controlled switches. U.S. Pat. No. 3,734,290 shows a washing machine safety door latch which mechanically senses the tub's rotation to prevent opening of the door until the rotation has stopped. U.S. Pat. No. 4,127,236 shows still another type of safety mechanical interlock mechanism used with food processing equipment to protect the operator. However, this safety mechanism does not contain provisions for the coasting time period of the drive motor.
Therefore, the need has existed for a relatively simple, inexpensive and durable safety device which prevents opening of an access door on power driven equipment until the power drive motor has coasted to a stop, and which prevents an operator from effectively bypassing the same.